The ferromagnetic monolayer Fe(110) on W(110)

Ferromagnetic order in the pseudomorphic monolayer Fe(110) on W(110) was analyzed experimentally using Conversion Electron Mössbauer Spectroscopy (CEMS) and Torsion Oscillation Magnetometry (TOM). The monolayer is thermodynamically stable, crystallizes to large monolayer patches at elevated temperatures and therefore forms an excellent approximation to the ideal monolayer structure. It is ferromagnetic below a Curie-temperatureT _c,mono, which is given by (282±3) K for the Ag-coated layer, (290±10) K for coating by Cu, Ag or Au and ≈210 K for the free monolayer. For the Ag-coated monolayer, ground state hyperfine fieldB _hf (0)=(11.9±0.3) T and magnetic moment per atom μ=2.53 μ_B could be determined, in fair agreement with theoretical predictions. Unusual properties of the phase transition are detected by the combination of both experimental techniques. Strong magnetic anisotropies, which are essential for ferromagnetic order, are determined by CEMS.     

Theoretical analysis of spin-resolved photoemission data from ferromagnetic Fe(001)

To interpret recent spin-, energy- and angle-resolved experimental photoemission spectra at photon energies ranging from 20 to 70 eV, one-step calculations on the basis of a non-relativistic Green function formalism have been performed together with a calculation of the corresponding bulk band structure and momentum-resolved layer-by-layer quasi-particle density of states. The theoretical spectra are in good agreement with the data. Individual features are explained in terms of bulk interband transitions and emission from a majority-spin surface resonance. Self-energy corrections are found to be important and in qualitative disagreement with recent microscopic theory.     

A LEED-AES study of the oxidation of Fe (110) and Fe (100)

Simultaneous LEED and AES observations have been used to study the initial stages of oxidation of the Fe(110) and Fe(100) single crystal surfaces at 300 K and 400 K and of a clean Fe polycrystal at 300 K. Accurate surface lattice spacings of the precursory oxide structures have been measured and attempts have been made to quantitatively evaluate the corresponding surface oxygen density.On the (110) single crystal surface the final structure is FeO-like with a lattice spacing 4% larger than that of bulk FeO. The transition to the FeO-like structure starts with a surface lattice expansion in the [11̄0] direction followed by an expansion in the [001] direction in order to accommodate the (111) face of the FeO-like structure. On the (100) single crystal face the oxygen and iron form an fcc (100) face which initially contracts and then expands with increasing oxygen doses. The structure formed at 300 K is spinel-like but heat treatment causes a transition to FeO(100).The changes of the surface unit cell dimensions are interpreted as the result of an interaction between adsorbate and substrate. This interaction is strongest in a direction parallel to the close packed rows of the substrate, making the corresponding periodicity of the adsorbate more resistant to lattice changes.In the case of the polycrystal a hexagonal structure was observed after oxygen adsorption with no simple relation to the oxide structures observed on the single crystals. The initial sticking coefficients in the interval 0–10^?5 torr sec ranged from 0.07 to 0.36 depending on temperature and crystal face observed. The latter dependence is explained in terms of the surface structure.     

Spin- and angle-resolved photoemission from ferromagnetic Fe(001)

For near-normal photoemission from ferromagnetic Fe(001) excited by linearly polarized synchrotron radiation, energy-resolved spin polarization and intensity distribution have been measured at 60 eV photon energy. Calculations using a one-step theory of photoemission consistently reproduce. the present spin-resolved data as well as earlier spin-averaged measurements. The quasi-particle exchange splitting deduced from the data is 2 eV. The agreement with band structure calculations is suggested to be coincidental due to a compensation of real and imaginary self-energy corrections.     

Angle-resolved photoemission from CO on Fe(110)

We report normal emission, angle-resolved photoemission measurements of the system p (1×2)Co-Fe(110) at several photon energies. The lower bind-energy feature, commonly attributed to emission from both the 1π and 5σ molecular orbitals of CO, shifts from 6.9 to 8.1 eV (relative to the Fermi energy) as the photon energy is increased from 21 to 30 eV. We attribute this dispersion to a large variation in cross section for photoionization of the 1π- and 5σ-derived orbitals and use symmetry selection rules to Identify the 6.9 eV component as the 1π and the 8.1 eV component as the 5σ. This ordering is reversed from the gas phase ordering.     

Theoretical study of relaxation at the (110) Ge-GaAs interface

A simple relaxation of the Ge-GaAs (110) interface is studied theoretically using self-consistent pseudopotentials. The relaxation is one in which the distance between the Ge and GaAs atomic planes at the interface is increased by 20%. Band edge discontinuities are found to be insensitive to this type of relaxation in spite of large distortions of the electronic charge density.     

Surface magnetism of Fe(110) from polarized electron scattering

Exchange- and spin—orbit-induced scattering asymmetry spectra of polarized slow electrons from the ferromagnetic Fe(110) surface have been calculated by dynamical theory and found to agree with recent experimental data taken at room temperature. Comparison of exchange asymmetry spectra, obtained for various interaction and layer-dependent magnetization models, with the data implies firstly an enhancement of the surface magnetization by about 30% with respect to the bulk, and secondly the importance of spin-dependent localized inelastic electron—electron scattering processes.     

Potassium adsorption on Fe(110)

LEED, AES, UPS and XPS were used to study submonolayer coverages of potassium on Fe(110). At room temperature the maximum potassium coverage is characterized by a LEED superstructure. This LEED pattern is interpreted as being due to a hexagonal close-packed K layer on Fe(110), resulting in a maximum atom density of 5.3 × 10¹⁴ cm^?2, i.e.θ k = 0.31. The work function change and the shift of the K(2p) and K(3p) core levels with potassium coverage indicate a charge transfer from potassium to iron at low potassium coverages.     

高能电子束韧致辐射特性的理论研究

高能电子束轰击金属靶会产生韧致辐射X射线,为优化韧致辐射X射线品质,需要研究如何获取最佳辐射效率等韧致辐射规律.结合理论分析,并采用MCNP/4C对10,20?MeV电子的韧致辐射规律进行了模拟研究.讨论了不同靶材料产生的韧致辐射效率、角分布、能谱分布、准直锥孔内辐射效率等问题.通过对不同靶材料韧致辐射的模拟研究,给出了不同厚度靶与光子效率、注量分布、出射电子与角分布的关系与规律.由此得到不同靶材料对于10,20?MeV电子在最优韧致辐射效率下的一些边界条件与规律. 关键词： 韧致辐射 最佳效率 角分布 能谱     

H2O分子在Fe(100), Fe(110), Fe(111)表面吸附的第一性原理研究

采用第一性原理研究了H2O分子在Fe(100),Fe(110),Fe(111)三个高对称晶面上的表面吸附.结果表明,H2O分子在三个晶面上的最稳定结构皆为平行于基底表面的顶位吸附结构.H2O分子与三个晶面相互作用的吸附能及几何结构计算结果表明H2O分子与三个晶面的相互作用程度不同,H2O分子与Fe(111)晶面的相互作用最强,其次是Fe(100),相互作用最弱的是Fe(110)表面,而这与晶面原子的排列密度相关.吸附体系的电子结构计算结果也得出了相似的结论.同时电荷布居分析表明,H2O分子与Fe表面相互作用时,O原子与基底原子之间的电荷交换使基底Fe原子表面带负电,导致表面电位降低,也促使Fe表面更易于发生电化学腐蚀反应.     

Magnetism of ultrathin Fe(110) films

Application of conversion electron Mössbauer spectroscopy (CEMS) to structural and magnetic analysis of ultrathin films and their interfaces is reviewed. Fe(110) films were prepared on W(110) under UHV conditions and analyzed in situ. CEMS provides detailed information on the mode of growth and film structure and on magnetic hyperfine fields, B _hf. Local structure of B _hf across the film is discussed in relation to modifications of magnetic order caused by the finite (including monolayer) film thickness and by the electronic structure of the interface.     

Desorption of H2 from W(110) and Fe covered W(110) surfaces

The kinetics of H₂ desorption from H/W(110) and H/Fe₁/W(110) were studied by measuring work function changes Δø vs time at a number of temperatures. Combination with previously determined Δø vs coverage data and differentiation at various fixed coverages gave rate vs T data from which activation energies of desorption could be obtained. E vs coverage results agree well with previously determine ΔH_des results. In the case of H/Fe₁/W(110) this includes a rise from 20 to 30 kcal mol⁻¹ of H₂ at H/Fe = H/W > 0.3. Plots of rate −dθ/dt vs θ (θ being coverage in units of H/W) vary much more steeply than θ² at most coverages for both systems. The θ dependence can be explained almost quantitatively in terms of the variations of ΔH_des and surface entropy S_s with coverage, by assuming that rates of desorption are equal to the equilibrium rates of adsorption. The latter can be formulated thermodynamically, except for a sticking coefficient, s. Values for s(θ, T) can also be obtained and show relatively little temperature dependence.     

Phonons at the Fe(110) surface

The in-plane density of phonon states of clean Fe(110) surface was measured separately for the first, second, and further atomic monolayers using nuclear inelastic scattering of synchrotron radiation. The results show that atoms of the first layer vibrate with frequencies significantly lower and amplitudes much larger than those in the bulk, and that vibrational spectra along two perpendicular in-surface directions are different. The vibrations of the second layer are already very close to those of the bulk. The good agreement of the experimental results and the first-principles calculations allows for detailed understanding of the observed phenomena.     

Interaction of carbon dioxide with Fe(110), stepped Fe(100) and Fe(111)

The adsorption of CO₂ on single crystal surfaces of Fe(110), regularly stepped Fe(110) and Fe(111) in the temperature range between 77 and 340 K was studied by means of He(I) UPS and measurements of the change in work function. The smooth Fe(110) face proved to be completely inactive with respect to CO₂ adsorption. On a stepped Fe(110) and an Fe(111) face CO₂ is adsorbed at 77 K in the form of a linear molecule and in the form of a species the nature of which is not yet clarified. This latter form is predominant at 140 K. With increasing temperature decomposition into CO and O and finally into C and O takes place.     

Periodic lattice distortions in epitaxial films of Fe(110) on W(110)

The epitaxial growth of Fe(110) on W(110) at 500 K is analyzed using LEED and AES. Frank-van der Merwe growth is established by AES. According to LEED, pseudomorphism occurs up to θ = 1.64, where every W atom of two topmost W layers is just covered by exactly one Fe atom. For 2?θ?9, characteristic reflection-multiplets are observed, symmetric about the basic Fe(110) reflections, which are interpreted in terms of periodic lattice distortions. The latter are caused by interaction with the misfitting W substrate.     

First principles study of behavior of helium at Fe(110)–graphene interface

Recently,metal-graphene nanocomposite system has aroused much interest due to its radiation tolerance behavior.However,the related atomic mechanism for the metal-graphene interface is still unknown.Further,stainless steels with Fe as main matrix are widely used in nuclear systems.Therefore,in this study,the atomic behaviors of point defects and helium(He) atoms at the Fe(110)-graphene interface are investigated systematically by first principles calculations.The results indicate that graphene interacts strongly with the Fe(110) substrate.In comparison with those of the original graphene and bulk Fe,the formation energy values of C vacancies and Fe point defects decrease significantly for Fe(110)-graphene.However,as He atoms have a high migration barrier and large binding energy at the interface,they are trapped at the interface once they enter into it.These theoretical results suggest that the Fe(110)-graphene interface acts as a strong sink that traps defects,suggesting the potential usage of steel-graphene with multiply interface structures for tolerating the radiation damage.     

Laser induced thermal desorption of carbon monoxide from Fe(110) surfaces

Thermal desorption of CO is induced by bombarding an Fe(110) surface with pulses of a neodymium glass laser. The maximum amplitude of the desorption signal is recorded by a mass spectrometer as a function of the laser pulse intensity and of the CO coverage for both single pulses and sequences of pulses. Since the half width of the laser pulses is only 30 ns the shape of the desorption signal is mainly determined by the time-of-flight of the desorbed particles. There is strong evidence that the latter obey a Maxwell-Boltzmann distribution of temperature T_d, identical in the low temperature range with the maximum surface temperature T_s. Above T_s = 600 K, however, T_d is smaller than T_s. The experimental observations are analyzed successfully with the first order rate equation for desorption.